Modern electric vehicles generally allow two charging modes. A vehicle has an on-board charging device for charging at a conventional AC voltage or three-phase socket, said charging device both performing the required conversion to direct current and controlling the respective charging operation. However, this AC charging mode is extremely restricted in terms of charging speed on account of the connection power available, which is generally not more than 16 A or 32 A, and on account of the installation of the charging device with sufficient power. This results in modern electric vehicles having charging times of several hours for every 100 km.
On account of the high charging times for AC charging, DC charging with DC voltage has been developed. In contrast to AC charging, in this case the corresponding vehicle does not have its own charging device. Instead, the charging post outside the vehicle carries out the charging process and likewise forms voltage and current as is necessary for charging a respective vehicle battery. DC charging lines provided accordingly are connected directly to corresponding poles of the high-voltage battery in the vehicle during the charging process. The powers of DC charging stations are currently up to 50 kW. However, charging powers in the region of more than 300 kW would be desirable in order to surpass charging speeds of more than 20 km/min. Furthermore, a charging voltage of up to 1000 V should be sought, in order to achieve a corresponding charging speed. Recharging during travel could thus be brought to orders of magnitude that customers are used to when filling up vehicles with internal combustion engines.
Exemplary details regarding DC charging and the corresponding processes in the charging system and in the vehicle are described in the current version of DIN EN 61851 as of the filing date of the German priority application, for example, which is incorporated fully herein by reference.
To enable higher charging speeds, various motor vehicle manufacturers are planning to enhance their vehicles from the previously usual 400 V charging technology to at least 800 V charging technology, which can correspond approximately to a working range of 600 V to 950 V, sometimes even 420 V to 980 V, on account of the voltage that is dependent on the state of charge and a certain variance in one configuration of the vehicle battery. A higher power can be transferred given the same current on account of the high voltage. It is therefore possible, by way of the charging duration, to solve one of the main problems of electrically operated vehicles.
However, not all manufacturers and vehicles will convert or be converted to 800 V charging technology. Nevertheless, a charging post and hence the electronics system used therein should be capable of being able to charge all vehicles that are to be charged, as far as possible.
The charging process is not constantly operated at a fixed voltage, even in 800 V vehicles. A vehicle is connected and the vehicle sets the voltage of the post initially approximately to its own battery voltage in a similar manner to a laboratory voltage source. The vehicle then connects the set voltage to the battery, sets a current limit on the charging post and sets the voltage limit, generally to a value between its own battery voltage and the charging connection voltage. The vehicle delivers the settings to the charging post by means of digital communication. There are therefore two limits, wherein the lower limit at the particular time dominates the regulation. In the current-regulation range, which dominates right at the start and in the middle of the charging process, the voltage assumes values significantly above the maximum. As a consequence, the power electronics system of the charging post has to cover a very high voltage range.
However, since the power electronics system cannot raise the current to the same extent at the same time on account of limits of the components comprised by the power electronics system, in particular the semiconductors and the inductances, the charging power undesirably drops in this range. Conventional vehicles with a low voltage therefore cannot be charged with the peak power of the power electronics system. Empty batteries must likewise first increase in voltage until the peak power is achieved on account of the maximally constant current, even though the batteries allow the highest charge currents and powers specifically at average states of charge.
In practice, it is very important, on account of safety for the vehicles and for the users, that the high-voltage connections of each charging point have to be isolated from all other charging points as well as from ground and the respective grid connection. Until now, only individual charging posts or correspondingly individual charging points that have fully distinct grid connections and act independently of one another have therefore been used in practice. Although there are also designs according to which a plurality of charging posts are combined, these designs entail only very minor simplifications in the construction and hence only limited cost savings compared to a solution having fully independent charging posts. One of these designs provides for the charging posts to have a common DC link, which consequently also has the same electrical potential for all charging points, and dedicated DC/DC converters for each charging point, said DC/DC converters having to be DC-isolated in turn for the abovementioned isolation.
It is further known to provide DC isolation not in the power electronics system itself but in a transformer having a plurality of taps that is to be connected to the power electronics system.
However, this approach does not solve the problem mentioned at the outset of a decreasing power at charge voltages under the peak voltage either.
U.S. Pat. No. 7,609,037, which is incorporated by reference herein, and an article by A. Lesnicar, R. Marquardt, “An innovative modular multilevel converter topology suitable for a wide power range” Power Tech Conference Proceedings, 2003 IEEE Bologna, vol. 3, p. 6 disclose circuits for a modular multilevel converter MMC, which allows switching over between series interconnection and bypass interconnection of individual modules and is incorporated by reference herein. DE 10 2010 052 934 and DE 10 2011 108 920, which are incorporated by reference herein, and S. M. Goetz, A. V. Peterchev, T. Weyh (2015) “Modular multilevel converter with series and parallel module connectivity: topology and control”, IEEE Transactions on Power Electronics, vol. 30, pp. 203-215, which is incorporated by reference herein, describe modular multilevel converters having series and parallel connectivity (MMSPC), which further allow switching over from parallel to series interconnection of modules.
However, none of these described circuits is suitable for DC voltage conversion in order to be able to charge all vehicles that are to be charged, as far as possible. Multiphase DC/DC converters are known. U.S. Pat. No. 6,628,106, for example, thus describes a multiphase DC/DC converter with combined current and voltage regulation. U.S. Pat. No. 6,995,548, which is incorporated by reference herein, describes a combination of a plurality of phases with different electrical parameters, for example rated current, in order to optimize the quality of the current at the output. U.S. Pat. No. 7,596,007, which is incorporated by reference herein, describes a multiphase DC/DC converter and associated inverter having DC isolation in the form of what is known as an LLC component. All of these circuits comprise hardwired DC/DC converters, which generate the respective output voltage, in particular in parallel. None of the described circuits is suitable for the abovementioned problem from the field of charging technology. None of the mentioned circuits is capable of drawing power from an AC grid, preferably a medium-voltage grid, and/or of switching over between voltage and current by reconfiguring modules or circuit components.
DE 10 2012 212 291 A1, which is incorporated by reference herein, describes an apparatus for rapid electrical DC charging, wherein a DC/DC regulator module has a DC/DC step-down module without DC isolation and has a DC/DC resonance converter module for DC isolation.
WO 2013/117425 A1, which is incorporated by reference herein, discloses a modular converter for a charging station, said modular converter comprising at least two charging modules connected in parallel with one another.
Against the background of the prior art, it was accordingly an object of the present invention to provide a possibility of being able to carry out a charging process in an electrically operated vehicle with the maximum possible power, even in a low voltage range, and of ensuring, in the process, DC isolation between the vehicle and the external grid at the same time.